Severn Bridge: All wrapped up

04 April 2008

The Highways Agency has delayed corrosion on the Severn Suspension Bridge with a dehumidification system from Laing O’Rourke and its partners, Munters and Faber Maunsell. The cable dehumidification system on the centre spans of the bridge, which joins England and Whales, is claimed to be the first operational system of its kind in the UK.

Severn Bridge
Severn Bridge

Using a polymer based material, the project involved wrapping the bridge’s main cable to create a homogenous seal and to install a dehumidification plant room within the bridge. The plant drives dry air into the main cable to reduce the level of humidity and suspend, or delay, the progression of corrosion of the cable strands.

The polymer based material, supplied by D S Brown of North Baltimore, was spirally wound onto the main cable and heat shrunk into position, creating a seal between each spirally wound section and a tight fit onto the main cable itself. To overcome the problem of heat transfer between the heating blanket and the irregular shape of the main cable Laing O’Rourke wrapping contractor, BASE Structures, developed a heating blanket incorporated in an air cushion that, when pressurised, formed itself onto the surface of the main cable whatever the irregularity.

A simple solution for access was provided by ALPS, which supplied twin access cables that spanned either side of the main cable. This boosted safety on a project where the majority of the work was at a height above and alongside the M48.

The dehumidification system was designed and installed by Munters.

For injection into the cable the external wire wrapping was removed from around the main cable in three locations on both centre span and main cables. Wedges were inserted into the cable strands to open up a path for the air flow, 16 Zinc wedges were installed in pairs around the main cable at each location. Stainless steel sleeves, fabricated by Jordan Engineering, formed a chamber round each of the locations where the main cable had been wedged open and provided an interface connection for the air flow pipework.

Air flow was provided from the plant room through the bridge structure and attached to the hanger cables up to the injection points in HDPE pipework, electro-fusion jointed and installed by Laing O’Rourke.

The plant room and dehumidification system was designed and commissioned by Munters. The plant room is inside the actual structure of the bridge and all the components, including the dehumidifier, were manufactured in sections to enable them to be lowered through the hatch, and then built inside the actual bridge.

There were 72 separate sections, which took six people an entire day to transport them to the actual site within the bridge. It then took three people a further five days to construct the plant room, complete with the dehumidification system.

The bridge is listed and the necessary approval had to be sought to enable several holes to be drilled in the structure in order for the necessary ductwork to be installed for the dehumidification system.

Monitoring equipment, linked to the desiccant dehumidifier, records the relative humidity, temperature and air pressure going into the cables. It also records the relative humidity, temperature and velocity of the air coming out of the cables. This ensures that the dehumidification system is working to its optimum capacity. The system has been designed to control the conditions at 20 per cent relative humidity in the plant room, which is then blown up the cables, via the large fans in the plant room.

The system is designed around the desiccant drying rotor, which is the moisture-absorbing component at the heart of the dehumidifier. Air to be dehumidified, in the cables, is drawn into the plant room and then through the rotor in the desiccant dehumidifier, where moisture is absorbed and the resulting dry air is delivered back to the cables. Simultaneously a separate air flow is heated and drawn through the remaining sector of the rotor. The air removes the moisture from the rotor and is discharged to the atmosphere.

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